In a welded steel pipe, since the quality of a welded portion is very important, an on-line flaw detection is generally performed to a welded portion by an ultrasonic angle beam technique in a manufacturing process. This method is such that an ultrasonic wave is caused to be obliquely incident on a inspection surface of a member to be inspected (hereinafter, referred to a material under test) to detect an inner and outer surface defect and an internal defect of the material under test from a reflected wave reflected on the defect. Ordinarily, in, for example, an electric resistance welded steel pipe, a reflection method of using an ultrasonic beam having a refraction angle of 45° at frequency of 5 MHz is used, and a defect having a size of the order of millimeter, for example, lack of penetration, burn-through, welding crack due to inclusions and the like is detected.
In contrast, recently, strict quality is required to a welded steel pipe, and it is required to detect a defect smaller than a conventional defect. For example, defects such as a cold joint defect and a minute penetrator are penetrated in a electric resistance welded steel pipe and defects such as a blow hole and the like are generated in a laser welded steel pipe, and these defects have a very small size of several tens of micron meters to several hundreds of micron meters. Further, there is a possibility that these defects are generated in any locations from an inner surface to an outer surface along a welding line, and the point on which an ultrasonic beam is incident may be different from a point to which it returns depending on a defect position. Since there are many cases in which defects can not be detected by conventionally practically used ultrasonic flaw detection methods due to these affects, a technology for detecting a defect more accurately is required.
The following conventional technologies are disclosed heretofore as a method of detecting a minute defect of a welded steel pipe. Patent Document 1 improves a detect resolution to a penetrator so that a point focus type probe having a frequency of 8 MHz or more can be used in a angle beam technique. Patent Document 2 improves a detect resolution by forming a focus beam by an array probe so that a blow hole can be detected by scanning from the inner surface side to the outer surface side of a welded portion by a sector scan.
Further, Patent Document 3 detects a cold joint defect formed of impurities as a group of minute FeO having a size of several micron meters or less by causing an ultrasonic wave having a frequency of 25 MHz or more to 500 MHz or less to be incident on a welded portion from the outer surface side of a pipe at an incident angle of 0° or more to 20° or less. Further, Patent Document 4 detects a blow hole of 0.1 mm or more by using a plurality of point focus type probes having a frequency of 20 MHz-80 MHz and disposing them so that a focusing position from just above a seam has a pitch of 3 mm or less.
However, the technologies disclosed above still have the problems described below. In the method of Patent Document 1, it has a problem in that since a focused ultrasonic beam has a narrow beam width, an equipment cost is increased because many channels are necessary to perform a flaw detection so that all the regions of a welded portion in a depth direction (wall thickness direction of a steel pipe) are not missed, and further a position adjustment and the like are very troublesome when a pipe size is changed. Further, when a defect shape is not a blow hole shape but a plane shape such as a penetrator and a cold joint defect and a defect is located inside of a wall thickness, it is difficult to detect the defect because a reflected wave travels in a direction different from an incident direction.
Since the method of Patent Document 2 can be performed using one array probe and setting can be made electronically when a size is changed, the former problem of Patent Document 1 can be solved. However, the latter problem thereof still remains unsolved.
When a defect shape is a plane shape as described above, since an upset is applied to, for example, the seamed portion of a electric resistance welded steel pipe, a defect has a very thin width of 100 μm or less when it is viewed from just above a seam. Accordingly, even the methods of Patent Documents 3 and 4 are difficult to detect the defect actually in many cases because a reflected wave from the defect is very weak. Further, a problem arises in that since an area of about 1-2 mm in the vicinity of a surface echo is made to a dead zone by the reverberation of the surface echo, when a defect is positioned in the vicinity of an outer surface, the defect cannot be detected.
As described above, it is difficult for the technologies, which detect a minute defect of about several hundreds of micron meters generated in the welded portion of a welded steel pipe in a pipe axis direction, to cope with the recently required strict quality control, and thus there is desired to develop a technology for solving these problems.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 60-205356    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 11-183446    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 61-111461    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 7-35729    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 4-274756    [Non Patent Document 1] “Ultrasonic Flaw Inspection Series (II), Ultrasonic Flaw Inspection Method of Welded Steel Pipe” The Iron and Steel Institute of Japan, 28 to 31 pages, 1988